The proposed clinical trial shall address the utility of inhaled iloprost for the prevention of lung cancer. Iloprost is a promising agent for the chemoprevention of lung cancer - a prior clinical trial showed reversion of bronchial dysplasia (a lung cancer precursor lesion) with oral iloprost. Since oral iloprost is no longer available, the goal of the current study is to determine if two different schedules of inhaled iloprost are tolerable and whether they modulate biomarkers of lung carcinogenesis. The trial is a phase I trial of two different dose levels of iloprost, 5 micrograms (ucg) four times daily (QID) or twice daily (BID) given for 2 months to former smokers at the University of Colorado, Denver (Denver VA Medical Center). In each of the two dose levels, participants will be randomized to iloprost or matched placebo and will undergo pre- and post-treatment bronchoscopies with biopsies so that tissue-based biomarkers can be examined